1. Field of the Invention
The present invention relates generally to a device for controlling the reverse rotation of a motor and a method of judging the time point where a motor actually begins to rotate in the reverse direction, and more particularly, to a device for controlling the reverse rotation of a motor capable of accurately detecting, when the motor begins to rotate in the reverse direction, the time point where the motor begins to rotate in the reverse direction and a method of judging the time point where a motor is actually begins to rotate in the reverse direction.
2. Description of the Prior Art
For example, a motor for driving an optical system for illuminating and scanning a document in a copying machine, a facsimile or the like or a motor for driving an automatic document feeder mounted on a copying machine is rotated in a first direction to drive the optical system or a document conveying belt of the automatic document feeder in the first direction, while being rotated in a second direction at predetermined timing to drive the optical system or the document conveying belt in the second direction opposite to the first direction.
In the driving of the optical system and the driving of the document conveying belt, it is necessary to accurately control the position where driving is reversed. Therefore, it is necessary to detect the time point where the motor which is a driving source is actually switched from the forward rotation to the reverse rotation.
Meanwhile, a rotating motor has inertia. Even if a reverse rotation instruction signal is applied, therefore, the motor is not immediately switched from the forward rotation to the reverse rotation. There is a time lag between the time when the reverse rotation instruction signal is applied and the time when the motor is actually switched from the forward rotation to the reverse rotation. In the motor for driving an optical system or the motor for driving a conveying belt of an automatic document feeder, therefore, it is impossible to carry out control with high precision unless the time lag between the time when the reverse rotation instruction signal is applied to the motor and the time when the motor is actually switched from the forward rotation to the reverse rotation is accurately detected and reflected for control.
Examples of the conventional reverse rotation control of, for example, a motor for driving an automatic document feeder include control for detecting the time when a motor begins to rotate in the reverse direction which is described in Japanese Patent Laid-Open Gazette No. 174538/1986. In the control described in the prior art document, output pulses of an encoder connected to the motor are detected and the difference between time t1 when a reverse rotation instruction signal is applied to the motor and time t2 when an output of the encoder does not vary is found, thereby to detect a time lag T between the time t1 when the reverse rotation instruction signal is applied to the motor and the time t2 when the motor actually starts to be rotated in the reverse direction.
Furthermore, the control in the above described prior art document may be replaced with, for example, control using a two-phase type encoder outputting pulses 90.degree. out of phase with each other in synchronism with the rotation of an axis of rotation of a motor, thereby to make it possible to accurately detect time when where the motor begins to rotate in the reverse direction.
In the two-phase type encoder outputting pulses 90.degree. out of phase with each other, A-phase and B-phase pulses are generally outputted by the rotation of the encoder, as shown in FIG. 8A. There is a 90.degree. phase difference between the A-phase pulses and the B-phase pulses. Therefore, at the time of the forward rotation of the motor, if the level of a B-phase signal is detected at the time point where the A-phase pulses rise, the level of the B-phase signal is, for example, high, as shown in FIG. 8B.
On the other hand, at the time of the reverse rotation of the motor, if the level of a B-phase signal is detected at the time point where the A-phase pulses rise, the level of the B-phase signal is, for example, low, as shown in FIG. 8C. Therefore, it is possible to accurately detect the forward rotation or the reverse rotation of the motor by detecting the level of the B-phase signal at the time point where the A-phase pulses rise.
In the control method described in Japanese Patent Laid-Open Gazette No. 174538/1986 out of the above described conventional control methods, a time lag T from the time when the reverse rotation instruction signal is outputted to the time when the actual reverse rotation is detected is relatively long, and the time point where the output of the encoder does not vary cannot be accurately detected. Therefore, this control method is not superior in precision as a control method of detecting the time point where the motor actually begins to rotate in the reverse direction.
On the other hand, in the control method using the two-phase type encoder outputting pulses 90.degree. out of phase with each other, it is possible to accurately detect the time point where the motor actually begins to rotate in the reverse direction. However, the two-phase type encoder is high in cost. Moreover, the number of signal lines from the encoder must be two because the encoder is of a two-phase type. Accordingly, a device for controlling the reverse rotation of a motor is high in cost if the two-phase type encoder is used.